If it's a motorised valve problem, and not just the motor there are also auxiliary switches in the head operated by the motor/gearing, then it's best to change the motor head complete so as to make sure the issue is not the switches.
Yes it costs more but gives reliability that just changing the motor cannot provide.
I'm not sure of the wiring on this boiler (sorry retired 5 years ago and find I can't remember the little details any more!) but could the switches in the motorhead control the firing of the burner for HW demand just like the normal HW cylinder stat arrangement? Those with a more modern finger in the pie will advise I'm sure.
Yes it costs more but gives reliability that just changing the motor cannot provide.
I'm not sure of the wiring on this boiler (sorry retired 5 years ago and find I can't remember the little details any more!) but could the switches in the motorhead control the firing of the burner for HW demand just like the normal HW cylinder stat arrangement? Those with a more modern finger in the pie will advise I'm sure.